GB816197A - Improvements in or relating to an image transforming apparatus - Google Patents

Abstract

816,197. Television. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. Oct. 11, 1957 [Oct. 23, 1956], No. 31796/57. Class 40 (3). In a television system for connecting from one scanning standard to another and employing a " writing-reading " type of storage tube, of the induced conductivity type, spurious signals in the output signals of the tube (the " read " signals) due to cross-modulation between the " reading " and " writing " beams are eliminated by algebraic subtraction of signals derived from both the target electrode and from the collector electrode to form the desired output signals. This results in cancellation of the spurious signals since such signals occur on the target and collector electrodes with the same polarities whereas the desired output signals occur on these electrodes with opposite polarities. In the embodiment of the invention shown in Fig. 1, the target electrode 4, one side of which is scanned by a high-velocity beam (the " writing " beam) produced by the electron gun 2 and the other side of which is scanned by a comparatively low-velocity beam (the " reading " beam) produced by the electron gun 3, supplies a signal U1, the polarity of which is negative during the " writing periods and positive during the " reading" periods to a circuit 25 in which it is algebraically subtracted from the signal U2 derived from the collector electrode 5, and the polarity of which is negative during both the " reading " and " writing " periods. The output of the circuit 25 then consists of a signal proportional to the algebraic difference between the signals U2 and U1 which occur during the " reading " periods since such signals are of opposite sign and are added absolutely whereas the signals which occur during the " writing " periods, and which include the spurious signals, cancel out, since they are of the same polarity and are made to have equal amplitudes in the circuit 25 by adjustment of the parameters of the latter. Such a circuit may include a double-triode valve (Fig. 3, not shown), although pentodes, transistors and other types of adding and subtracting circuits may be employed. One application of the invention referred to is the reproduction of a television type display from the output signals of the tube when the input signals applied at terminal 27 are derived from a radar P.P.I. system.